ADD support to allow omp single to be CG-ed by the OMPBuilder. This
also uses the OMPBuilder to generate __kmpc_copyprivate calls.
Because the OMPBuilder currently is not the default way to CG OMP code in clang, which means the old way needs to keep working. So, we cannot delete or modify anything, we can only add the OMPBuilder specific code, and condition its usage on whether -fopenmp-enable-irbuilder flag has been issued, or not.